Improved forage sugar concentration seen with management and selectionWritten by Natasha Wheeler
The growth of microbes in the rumen depends on both the energy and protein content of forage that a cow consumes, according to Gilles Belanger, research scientist at Agriculture and Agri-Food Canada in Québec City.
“In a typical forage, we often have an imbalance between the energy and the protein that is available to the microbes. What limits the microbial growth is the available energy,” he explains.
Increasing sugar concentration of forages can create more available energy for the ruminant microbes.
“If we increase sugar concentration, we are likely to have better intake from the animals, and that’s been shown both in the U.S. and the United Kingdom (UK). The work in the UK has also shown that if we feed high-sugar forages to dairy cattle, we improve nitrogen efficiency in cows,” he explains.
In other words, he clarifies, forages with a higher concentration of sugars are likely to improve ruminant performance as well as reduce nitrogen losses to the environment.
Morning versus afternoon
To investigate how sugars can be retained, Belanger and his team designed a study to measure the levels of starches and sugars in forages throughout the day. Samples were taken from an hour after sunrise through the evening and tested for nutrient levels.
“As we go through the day, there is an increase in total amount of sugars, and the increase comes mostly from the starch. As the crop goes through photosynthesis it produces sugars faster than the crop can use them for growth, so there is an accumulation of sugars during the day,” he describes.
Maximum sugar concentrations were seen at the end of the afternoon, between approximately 11 and 13 hours after sunrise.
“If we want to increase sugar concentration in forage, probably the easiest thing to do is to cut our forages in the afternoon,” states Belanger.
Depending on the species of forage, sugar concentration was recorded to increase between two and four percentage points over the course of the day. For example, one species may have six percent sugars in the morning and eight percent by the end of the day.
Belanger and his team also looked at how sugar concentration was impacted after cutting, while laying out on the field.
“In both morning and afternoon cutting, sugar concentration decreased over time, and that’s normal because plants that are cut are still alive and continue to respire for a number of hours. They will use some of their sugars through respiration,” he explains.
But, looking at both cuttings at the end of the wilting period revealed that forages cut in the afternoon still retained more sugar content.
“At the end of the wilting period, when it’s time to ensile, we still have the same difference,” he says.
Swathing was another factor that Belanger and his team considered. Measuring samples that were left alone to dry and samples that had been swathed immediately, they discovered that swathing reduced the sugar concentration of the forages.
“Cutting forages without swathing maximizes sugar concentration in wilted forage when climatic and crop conditions favor rapid wilting,” he remarks.
Because microorganisms feed on sugars, differences were also seen in the fermentation process of silage made from the forages.
“We found that if we have a forage that is rich in sugars, that silage will have better conservation and better fermentation, and ultimately, we will also have a lower pH, a higher lactate concentration and a lower ammonia concentration. These are three characteristics that are significant in terms of characterizing silage quality,” he explains.
Belanger also notes that improved silage and residual starch concentration in sugar-enriched alfalfa could mean improved milk production in cows.
“For all of the forage species that we tested, there was an increase in sugar concentration throughout the day, but the increase varied among species,” Belanger continues.
The team looked at several grasses, including reed canary grass, smooth bromegrass, meadow bromegrass, timothy, tall fescue and Kentucky bluegrass, along with two legumes including alfalfa and red clover.
“Tall fescue and red clover tended to have higher concentrations,” he says. “There is a difference between species for sugar concentration, so selecting certain species could have an impact in terms of producing sweet forages.”
Lastly, Belanger also shares some preliminary data concerning genetic selection, where species of alfalfa were selected for high and low sugar concentrations and compared to a control alfalfa crop. They found that they were able to impact the varieties, which they found to be promising.
“That helped us put into perspective the improvements we can make through breeding and also through management,” he notes. “With genetic selection so far, we have been able to improve concentration by about one percent and we still have a lot of work to do in terms of genetic selection.”
To summarize, Belanger concludes, “Afternoon cutting, wide swathing and species selection can be used to increase forage sugar concentration by two to four percent.”